This invention relates to transporting and storing liquid ammonia and, more specifically, to limiting the risk of harm in the event of an accidental liquid ammonia spill. (As used in the specification and claims, the term "liquid ammonia" means any grade of ammonia existing in the liquid state that boils at or below ambient temperature and pressure, i.e., about 101.3 kpascal (about 1 atm) and about 20.degree. C. (about 68.degree. F.)).
Anhydrous liquid ammonia is used in massive quantities world-wide for many industrial and agricultural purposes. (As used in the specification and claims, the term "anhydrous liquid ammonia" means liquid ammonia that is at least about 98 weight percent ammonia and up to about 2 weight percent water. Under one commercial standard, anhydrous liquid ammonia transported by pipeline must be at least 99.5 weight percent ammonia. Anhydrous liquid ammonia is thus substantially water-free, but nevertheless often contains a small proportion of water, e.g., about 0.1 to about 0.5 weight percent, purposely included to inhibit corrosion of storage vessels and pipes.) A gas at ambient temperatures and pressures, anhydrous liquid ammonia is normally shipped and stored as a liquid, either in pressure vessels at ambient temperature, e.g., at about 20.degree. C. (about 68.degree. F.) and about 928.7 kpascal (about 120 psig), or in refrigerated vessels at ambient or nearly ambient pressure, e.g., at about -33.degree. C. (about -27.4.degree. F.) and about 104.8 kpascal (about 0.5 psig). It is transported in bulk in ships, barges, and railroad tank cars, and in tank trucks on public roads and highways. It is frequently stored in large quantities at industrial sites in populated areas and is frequently used as the working fluid in large refrigeration systems. It is now coming into wider use for the removal of NOx from flue gas at power generating stations in urban areas.
Anhydrous liquid ammonia is an extremely hazardous, toxic, and volatile material. In the event of an accidental discharge, it can cause immediate death to humans and animals and rapid death to trees and plants.
Both anhydrous liquid ammonia and very concentrated aqueous liquid ammonia display a deadly characteristic which substantially increases the risk of widespread injury and death in case of a spill. Specifically, upon sudden release to the atmosphere, as might occur in a train wreck or a traffic accident, the ammonia forms a cloud made up of an aerosol fog of liquid ammonia droplets. Unlike gaseous ammonia, which, though toxic, is lighter than air and quickly dissipates to harmless concentrations, the cloud can persist for a surprisingly long time, as long as several hours, before it finally disappears. The cloud is typically heavier than air and tends to drift along the surface of the earth, i.e., the ground or the surface of a body of water. The cloud moves with the wind and can sweep over a total area, i.e., a "footprint," much larger than the area covered by the cloud at any one moment. Contact with the cloud is instantly incapacitating, and a single breath can be fatal.
Substantial numbers of bulk shipments of anhydrous liquid ammonia routinely move through or near densely populated areas. It is estimated that an anhydrous liquid ammonia spill from a truck trailer (about 18,143.7 kg (about 40,000 lb) liquid ammonia capacity) would generate a cloud having an average lethal footprint of about 1173.6 are (about 29 acres), that is, an area of about 1173.6 are (about 29 acres) in which the concentration of ammonia would reach a lethal level, about 0.5 percent, before the cloud eventually dissipated.